1. Field of the Invention
The present invention relates to a propelling drive transmission system of a working vehicle having change speed gearing, and more particularly to a propelling drive transmission system operable in response to a change speed command to disengage a change speed clutch, shift the change speed gearing, and engage the change speed clutch again after the shifting operation.
2. Description of the Related Art
In the above transmission system for a working vehicle, for example, the change speed clutch is disposed downstream, with respect to a direction of drive transmission, of the change speed gearing which is shiftable by a hydraulic actuator. In response to a change speed command, the change speed clutch is automatically disengaged to disconnect the drive wheels from the change speed gearing. Thereafter the change speed gearing is operated by the actuator. Upon completion of the shifting operation, the change speed clutch is automatically engaged to enable drive transmission again.
To shift the above change speed gearing, a shift gear engaging one transmission gear must be caused to slide away from that gear and engage a desired transmission gear. At this time, the shift gear and desired transmission gear are brought into the same rotating rate before the shift gear engages the desired transmission gear. A typical example of change speed gearing achieving this is known as the synchromesh type. The transmission system including such change speed gearing has the following disadvantage.
When the change speed clutch is disengaged in response to a change speed command, a transmission shaft of the change speed gearing operatively connected to the change speed clutch, i.e. an output shaft of the change speed gearing, is placed in freely rotating state. However, a transmission shaft of the change speed gearing operatively connected to the engine, i.e. an input shaft of the change speed gearing, remains to be driven at a high rotating rate by the engine drive. At this time, a substantial difference exists in rotating rate between the input shaft and output shaft.
Thus, when the change speed gearing is shifted by the actuator, a considerable time is consumed before the output shaft is brought to the same rotating rate as the input shaft. This is one of the factors hampering high-speed operation of this transmission system.
As an improvement in the above situation, a transmission system is known from Japanese Patent Publication Kokai No. 4-39464, which includes a first change speed clutch, change speed gearing and a second change speed clutch arranged in the stated order downstream of an engine.
According to this system, as an actuator starts shifting the change speed gearing in response to a change speed command, the first and second clutches are disengaged. The first and second clutches are engaged again in response to completion of the shifting operation by the actuator. With this construction, the first clutch upstream of the change speed gearing is disengaged as well as the second, downstream clutch. Thus, when a shifting operation is effected, a transmission shaft of the change speed gearing operatively connected to drive wheels, i.e. an output shaft of the change speed gearing, and a transmission shaft of the change speed gearing operatively connected to the engine, i.e. an input shaft of the change speed gearing, are both placed in freely rotating state. Consequently, a reduction is expected in the time taken for the input shaft and output shaft to attain the same rotating rate when a shifting operation is effected, compared with the first-mentioned transmission system.
However, in this transmission system also, immediately after disengagement of the first and second clutches, the input shaft remains rotating at a high rate corresponding to the drive of the engine whereas the output shaft rotates at a lower rate corresponding to a running speed of the vehicle.
Thus, despite disengagement of the first and second clutches, a significant difference still exists in rotating rate between the input shaft and output shaft of the change speed gearing. Even though these shafts are in freely rotating state, a little time is consumed before the shafts are brought to the same rotating rate. A further improvement is required to realize a smooth and quick shifting operation.